Organic semiconducting molecules and polymers are promising components in emerging optoelectronic technologies, such as light emitting devices,1-3 flexible thin film transistors,4-6 and plastic solar cells.7-9 An important potential advantage, relative to inorganic systems, is the possibility to deposit device layers directly from solution and thereby to simplify fabrication methods. Physical organic chemistry principles can be used to tailor molecular properties relevant for function including emission color, ionization potential and electron affinity. However, the bulk behavior is more difficult to predict as the final organization in the solid is mediated by weak intermolecular forces and is therefore dependent on the deposition history and subsequent fabrication steps. Intermolecular order can control, for example, emission output,10 charge generation efficiencies in photovoltaic devices11-14 and the mobilities of charge carriers.15-17 The problem of predicting function from molecular design is exacerbated at metal/organic interfaces, where the molecular orientation of even a monolayer can have a substantial effect on the electric field required for charge injection.18-21
Electron injection from a metal electrode is an important longstanding area of interest within the scope of organic light emitting diodes (LEDs).22-24 In the absence of interfacial effects, the electron current is limited to a combination of Fowler-Nordheim tunneling and thermionic emission mechanisms where the barrier corresponds to the difference in energy levels between the cathode work function and the π*-band (lowest unoccupied molecular orbital) of the semiconducting polymer.25 Injection barriers, which are large for stable metals such as Al due to their high work function, increase the operational voltage and can lead to unbalanced hole and electron currents and low electroluminescence efficiencies.26-28 They are therefore an essential problem in the design of organic optoelectronic materials. Based on the considerations above, electron transporting/injection layers (ETLs) have been incorporated within organic LEDs.29-31 These layers reduce the electron injection barrier by a variety of mechanisms, including placing a dipole adjacent to the cathode,32,33 band bending using doped materials to create ohmic contacts34 or through a redistribution of energy levels via charge accumulation.35,36 Conjugated polyelectrolytes, i.e. conjugated polymers bearing pendant groups with ionic functionalities, have recently been used as effective ETLs.37-39 That these polymers are soluble in polar solvents offers a practical advantage to eliminate dissolution of nonpolar underlying layers.40 The presence of the ionic component can lead to long response times when ion motion and the resulting electric field redistribution play a fundamental role in improving injection.41,42 Conjugated oligoelectrolytes have been similarly used as ETLs to improve electron injection.43,44 The charge compensating counterions in both types of materials are an important ingredient in determining the final performance of the device. A recent study showed that tetrakis(1-imidazolyl)borate (BIm4) proved to be a particularly useful anion when coupled with either cationic conjugated polyelectrolytes or oligoelectrolytes.39-44 